So batch #6 is just about ready to be bottled. I've been noticing the same off-flavor inhabiting all of my brews, couldn't really put my finger on what it was (I'm brewing with extract and specialty grain). Tried different sanitizers, different extract, different styles, bought a grain mill, and started using bottled water, but to no avail. When my friend tried batch #5, he described the flavor as "metallic." So I start researching what causes that, and turns out aluminum pots supposedly can. Looking at it, I never researched enough about aluminum to prepare the pot. I never boiled any water in it prior to brewing, so I'm wondering how possible it is that I've brewed 6 batches of beer and never built up an oxide layer, maybe the wort isn't allowing anything to build up? If I go ahead and boil some water for about an hour now will I be good, even though the pot has been used quite a bit? Any help would be greatly appreciated, because this is getting pretty discouraging!!

I and many others use aluminum pots, so I don't think that should be a problem. Since this is affecting all of you batches, have you thought about testing your water itself? If you are using tap water, there could be high mineral content contributing off flavors. You could also try a batch using distiller water. Just a thought.